Trace transition metals were introduced in the catalytic decomposition of alkali hydrogen peroxide at va-rious pH values,and the corresponding decomposition kinetics and mechanism were investigated.The results indicate that the decomposition of hydrogen peroxide follows the quasi-first-order law in alkaline conditions,that the quasi-first-order rate constant of the alkali decomposition,kobs,increases with the pH value when 10.0≤pH≤11.6 but decreases when pH11.6,and that kobs reaches the maximum 0.0922 min-1 at 70℃ and pH11.6.Moreover,the effects of pH value and reaction time on the size of transition-metal colloidal particles were also discussed,finding that the average sizes of Mn ^2＋ and Fe ^3＋ colloidal particles increase with the reaction time and the pH value,respectively.Thus,it is concluded that the decomposition of alkaline hydrogen peroxide is due to the catalytic decomposition of transition metal colloids and the alkaline ionization of hydrogen peroxide.